Incorporating fibrous reinforcements, such as glass fibers and rock wool fibers, into polycarbonate resins is known to improve dimensional stability, heat distortion temperature, creep resistance, tensile strength and, most dramatically, elastic modulus. However, this always results in a serious deterioration in overall ductility, manifested in poor notched and unnotched impact strength as well as a decreased falling ball impact strength. Even small amounts of fibrous reinforcements have a serious effect on the ductility of polycarbonate. If it is sought to improve impact performance by adding conventional impact modifiers, such as selectively hydrogenated styrene-butadiene styrene block copolymers, then there is a detrimental effect on stiffness (modulus) and only a minor improvement in impact strength, in any event. It has been found that elimination of the adhesive bond between polycarbonate and fibrous reinforcing agents can be accomplished by burning off or otherwise using fibers free of conventional sizing or coupling agents. This does improve ductility, but only for relatively small fiber contents, e.g., up to less than about 10% by weight of sizing-free glass fibers in the polycarbonate--this is usually below the optimum amount.
The ductility of the compositions mentioned above decreases even further when linear polyesters are blended in the fiber-reinforced polycarbonates in order to establish outstanding chemical resistance. Polycarbonates are known to have a very limited resistance against environmental stress crack conditions.
It has now been discovered that the addition of poly C.sub.1 -C.sub.10 alkyl (or phenyl) hydrogen siloxanes to compositions comprising "pristine" (or sizing-free) fibrous reinforcements and polycarbonates-polyester admixtures, in which the fiber content exceeds even 30%, results in a tremendous improvement in falling ball (ductile) impact strength, and notched impact and unnotched impact strengths, too. These can be improved by several hundred percent with almost full retention of the elastic modulus.
The foregoing is altogether surprising in light of Alewelt et al., U.S. Pat. No. 4,147,707, who describe glass fiber reinforced polycarbonates with improved mechanical properties containing 0.5 to 5.0% of organopolysiloxane. While U.S. Pat. No. 4,147,707 states that both long and short glass fibers can be used, Col. 3, lines 22-50, it is specified that they must be "provided with a polycarbonate-compatible finish by means of suitable sizes" (Col. 3, lines 25-27). The patent makes no distinction between conventional silicones, like polydimethyl siloxanes, and those containing silicone-hydrogen bonding. Applicant finds superior results with unsized glass fibers, if a hydrogen-siloxane is selected, and then used in amounts below 1.0%, and especially below the 0.5% lower limit of Alewelt et al. The falling ball ductile impact with such specific hydrogen polysiloxanes is, as will be illustrated later, more than ten times greater than with the dimethyl-polysiloxanes used in Alewelt et al. Bialous et al., U.S. Pat. No. 3,971,756 is also relevant to the present invention, but only insofar as it shows that from 0.01 to about 5 weight percent of a polysiloxane having silicon-bonded hydrogens can be used to prevent dripping in flame retardant polycarbonate compositions. Although the amounts and types of hydrogen siloxanes suggested in U.S. Pat. No. 3,971,756 are within the limits employed herein, and the inclusion of fibrous glass is suggested, the need for sizing-free fibers to enhance ductile impact is not at all evident.
It is believed that the following conditions are essential herein:
(i) sizing agents (on the fibrous reinforcement or separately added) must be absent because these either evoke adhesive bonds between the matrix and fiber, or they prevent reactions between the hydrogen polysiloxane and the fiber, or both:
(ii) a very good dispersion of the fibers in the matrix is required;
(iii) for best combination of high modulus and creep performance, the addition of polysiloxane is preferably kept below 1.0% and, especially preferably, below 0.5%; and
(iv) the polysiloxane used must contain hydrogen silicon bonds.
Following the use, especially, of short glass fibers, additional advantages in improved isotropy and high surface quality are obtained. It is again reemphasized, that sizing agents must not be present to contribute to adhesive bonds between matrix and fibers, nor should they prevent reactions between the silicon-hydrogen bond-containing polysiloxane and the fibers. In practical terms this means that pristine fibers should be used. Using the factors mentioned above, the falling dart impact strength of a 20% short glass fiber-reinforced polycarbonate polyester blend can be increased from &lt;5 J to 80 J, while the unnotched impact bar increased from about 400 to about 800 J/m. The new composition has a desirable high modulus. These results are evident at surprisingly low levels of hydrogen polysiloxane. Substantially the same results are also obtained with other fibrous fillers, pristine or virgin, including rockwood-mineral fibers, carbon fibers, and the like.